PID Control of a Batch ph Process with a Strong Acid

Transcription

1 Exercise 1-1 PID Control of a Batch ph Process with a Strong Acid EXERCISE OBJECTIVE Familiarize yourself with the trial-and-error method for tuning your controller and apply this method when tuning a batch ph process using a strong acid. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Review of relevant control schemes Tuning with the trial-and-error method A procedure for the trial-and-error method. A complementary approach to trial-and-error tuning. ph Processes Control of ph. Batch and continuous processes. Batch processes Batch process example: paper making. DISCUSSION This exercise introduces three control schemes and puts them to use in a ph process loop. This allows a comparative analysis of the different schemes in terms of efficiency, simplicity, and applicability to various situations. An intuitive method to tune controllers is also presented. Review of relevant control schemes A controller in proportional mode (P mode) outputs a signal (: manipulated variable) which is proportional to the difference between the target value (SP: set point) and the actual value of the variable (: controlled variable). This simple scheme works well but typically causes an offset. The only parameter to tune is the controller gain (or the proportional band ( ) if your controller uses this parameter instead). A controller in proportional/integral mode (PI mode) works in a fashion similar to a controller in P mode, but also integrates the error over time to reduce the residual error to zero. The integral action tends to respond slowly to a change in error for large values of the integral time and increases the risks of overshoot and instability for small values of.thus, the two parameters which require tuning for this control method are (or ) and (or the integral gain, defined as ). A controller in proportional, integral, and derivative mode (PID mode) incorporates the three control actions into a single polyvalent and powerful control scheme. The addition of derivative action to the PI mode results in the capacity to attenuate overshoots to some extent, but adds the risk of instability if the process is noisy. Tuning a controller in PID mode requires careful adjustment of the proportional gain ( ), the integral time ( ), and the derivative time ( ) to properly address the control requirements of the process. In some circumstances, the controller Festo Didactic

2 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion output must not be zero when the error is null. In these cases, a bias (), also known as manual reset, must be set. The on-off control mode is the simplest control scheme available. It involves either a 0% or a 100% output signal from the controller based on the sign of the measured error. The option to add a dead band, available with most controllers, reduces the oscillation frequency and prevents premature wear of the final control element. There are no parameters to specify for this mode beyond a set point and dead band parameters. Note that it is possible to simulate an on-off mode with a controller in P mode for a large value of (or a very small). Tuning with the trial-and-error method The trial-and-error method of controller tuning is a procedure for adjusting the P, I, and D parameters until the controller is able to rapidly correct its output in response to a step change in the error. This correction is to be performed without excessive overshooting of the controlled variable. This method is widely used because it does not require the characteristics of the process to be known nor is it necessary to bring the process into a sustained oscillation. Another important aspect of this method is that it is instrumental in developing an intuition for the effects of each of the tuning parameters. However, the trial-and-error method can be daunting for inexperienced technicians because a change in tuning constant tends to affect the action of all three controller terms. For example, increasing the integral action increases the overshooting which, in turn, increases the rate of change of the error triggers an increase in the derivative action. A structured approach and experience help in obtaining a good tuning relatively quickly without resorting to involved calculations. A good trial-and-error method is to follow a geometrical progression in the search for optimal parameters. For example, multiplying or dividing one of the tuning parameters by two at each iteration can help you converge quickly toward an optimal value of the parameter. A procedure for the trial-and-error method The trial-and-error method is performed using the following procedure (also refer to Figure 1-19 and Figure 1-20 for PI control): 1. Set the controller to the desired mode: P, PI, PD, or PID. Follow the instructions to adjust each parameter relevant to this mode. Note that, if your controller allows, you can use the PID mode to perform any of the modes by simply setting the parameters to appropriate values (e.g., for PI mode). Adjusting the P action 2. With the controller in manual mode, turn off the integral and derivative actions of the controller by setting and respectively to the largest possible value and Festo Didactic

3 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion The controller gain is related to the proportional band:. If your controller uses the proportional band, start with a value of, and replace instructions to increase by a factor of two by instructions to decrease by a factor of two. 3. Set the controller gain to an arbitrary, but small, value, such as Place the controller in the automatic (closed-loop) mode. 5. Make a step change to the set point and observe the response of the controlled variable. The set point change should be typical of the expected use of the system. Since the controller gain is low, the controlled variable will take a relatively long time to stabilize (i.e., the response is likely to be overdamped). 6. Increase by a factor of 2 and make another step change to the set point. Observe the effect on the response of the controlled variable. The objective is to find the value of at which the response becomes underdamped and oscillatory. This is the ultimate controller gain. Keep increasing by factors of 2, performing a set point change after each new attempt, until you observe the oscillatory response. Once the ultimate controller gain is reached, revert back to the previous value of by decreasing the controller gain by a factor of 2. The P action is now appropriately set to add another control action if required. Adjusting the I action 7. Add integral action by setting the integral time at an arbitrarily high value. Decrease by factors of 2, making a set point change after each setting. Continue until you reach a value of, at which the response of the controlled variable becomes underdamped and oscillatory. At this point, revert back to the previous value of by increasing to twice its value. The I action is now set. If required, you can now proceed to the adjustment of the D action, if required. Adjusting the D action 8. Add derivative action by setting the derivative time at an arbitrarily low value. Increase by factors of 2, making a set point change after each setting. Continue until you reach the value of that gives the fastest response without amplifying the overshooting or creating oscillation. The D action is now set. Fine-tuning the parameters 9. Fine-tune parameters of the controller until the requirements regarding the response time and overshooting of the controlled variable are satisfied. Festo Didactic

4 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion With the controller in manual mode, turn off the integral and derivative actions. Set the controller gain to 1.0. Place the controller in automatic mode. Make a step change in the set point. Is the process response underdamped and oscillatory? No Increase the controller gain to twice its value. Yes Set the gain to halfway between the actual gain and the previous gain. Bring in the integral action by setting the integral time at a high value. Make a step change to the set point. Is the process response underdamped and oscillatory? No Decrease the integral time by a factor of 2. Yes Set the integral time to halfway between the actual time and the previous time. Bring in the derivative action by setting the derivative time at a low value. Make a step change in the set point. Is the process response longer and is overshooting amplified? No Increase the derivative time to twice its value. Yes Reduce the derivative time to obtain the fastest response without overshooting amplification. Fine-tune the controller to meet the response requirements. Figure 1-19.Trial-and-error tuning method. 22 Festo Didactic

5 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion A complementary approach to trial-and-error tuning Another, more visual approach is to use Figure 1-20 to assist in tuning your controller. The figure presents responses of a PI process to a step change for different combinations of parameters. A good tuning is shown in the center of the figure for optimal and parameters. The tuning in the center is not necessarily the most appropriate for all processes, but the response shown is a good target for a rough first tuning. The figure also shows responses for detuned parameters (both above and below the optimal and ). If you compare the response obtained for your system with the detuned responses in the figure, you will find the direction to change and/or to converge towards the center case. Changing the parameters by a factor of two at every step until you get very close to the optimal value is a good method to converge rapidly. Figure PID tuning chart. If required, derivative action can then be added to the control scheme by following step 8 of the trial-and-error method. Then, fine-tune the parameters to optimize the control and meet the specific requirements of your process. ph Processes A ph process is the sequence of operations required to maintain the ph of a solution (the controlled variable) to a chosen ph value (the set point). A ph probe and a ph transmitter are used conjointly to measure the ph of the solution. The Festo Didactic

6 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion measurement results are used to determine the action needed keep the controlled variable at the required value. Thus, the ph is also the measured variable. To compensate for any difference between the set point and the controlled variable, small quantities of acid or base are added to the solution. Control of ph The control of ph is essential in many industrial processes. This is especially true for processes where the chemical reaction rate is higher within a specific ph range. If a chemical reagent is added to a process when ph is not at the right value, the reaction may take too long or may not occur at all. In the food processing industry, ph is of the highest importance. An inadequate ph not only results in bad-tasting food, but can also influence the color, smell, and shelf life of the product. Controlling the ph can increase the quality of the final product, the safety of the process, and the performance of the business. For example, consider the chemical conversion process shown in Figure The purpose of the process is to combine a liquid reactant, A, and a gaseous reactant, B, to form a third liquid product, C. Reactants A and B are first charged into the reactor. A reaction takes place in the reactor causing the ph of the solution to increase. To obtain a maximum conversion of the reactants, ph loop 6 keeps the ph of the liquid in the reactor constant by adding acid to the solution maintaining the ph of the solution at an ideal value. The liquid product is then removed from the bottom of the reactor and passed through an evaporator. As it flows through the evaporator, the liquid product loses thermal energy causing its temperature to decrease and the reaction to stop. To ensure that the reaction is correctly stopped, temperature loop 7 keeps the temperature of the liquid product constant through speed variation of the evaporator blower. 24 Festo Didactic

7 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion Flow loop 5 Level loop 2 Gas purge Reactant A (liquid) Flow loop 1 Heater Acid Gas-liquid reactor Reactant B (gas) Pressure loop 3 ph loop 6 Evaporator Product C to separator Temperature loop 4 Temperature loop 7 Figure Chemical conversion process. This example illustrates how industrial processes often require a reaction to take place within a specific ph range. In this unit, you will perform PID control of a batch ph process using a loop similar to the one in Figure Batch and continuous processes Processes can be divided into two categories: batch and continuous. Batch processes are sequential and are typically used in the food, fine chemicals, and pharmaceutical industries. The paper industry also uses batch processes to make paper pulp. During the 1930s, most petroleum production processes, such as thermal cracking of crude oil, were batch processes. However, due to increased market demand, petroleum manufacturers now use continuous reactors to process materials indefinitely. In addition, bulk chemicals and Festo Didactic

8 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion fertilizers are mainly manufactured using continuous processes. Wastewater treatment plants also use continuous processes to safely recycle or dispose of water. Even though processes are classified as batch or continuous, most industries use processes that combine batch and continuous systems. High industry standards for both product quality and cost efficiency often require many batch processes to keep continuous control over at least one variable. Besides, almost all continuous manufacturing processes end with batch quality control testing, packaging, and shipping. Batch processes A batch process as defined by the Instrumentation, Systems, and Automation Society (ISA) SP88 committee, is a process that manufactures a finite quantity of material by subjecting measured quantities of raw materials to a sequential order of processing actions using one or more pieces of equipment. Most batch operations are usually completed within minutes or hours. To achieve batch control, a recipe is required. A recipe is the complete set of data and operations that uniquely defines the control requirements to produce a batch of a specific product. Using a specific recipe, a technician should have all the necessary information such as material quantities, order of processing activities, set points, mixing, etc. to make a determined quantity of a product. When fluids are involved in batch processes, only a specific quantity of reactants is treated in one run. The volume increase due to the addition of solid or liquid substances in the reactor is usually considered negligible. Also, there are no input or output flows into or out of the reactor. Batch processes are usually used to treat small or medium volumes. Because batch processes imply a beginning and an end, high efficiency is rarely achievable. When large volumes need to be treated and/or high-efficiency is required, continuous processes are usually favored. Figure 1-22 shows a simplified batch reactor for a ph process. Ideal mixing device Determined quantity of acid or base Figure Simplified batch reactor for a ph process. When chemical equilibrium is reached in this type of reactor, the steady-state is also reached. Therefore, if a batch ph process is at steady-state, it is also in chemical equilibrium; the concentrations of the species in solution have no net change over time. 26 Festo Didactic

9 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Discussion The following points summarize the main characteristics of this idealized batch ph process: a predetermined quantity of reactant is treated flow into and out of the tank is null negligible volume increase used to treat small or medium volumes the initial hydronium concentration is the concentration of is affected by the addition of an acid or a base medium-efficiency process chemical equilibrium and steady-state describe the same phenomenon Batch process example: paper making Paper making is a complex task that includes numerous chemical treatments, mixing sequences, and reprocessing operations. Both batch and continuous process control are required to carry out these tasks and obtain paper as the final product. Figure 1-23 illustrates a simplified version of a paper making process. A mix of softwood and hardwood pulp is used to make paper. Proportions of softwood to hardwood vary according to the desired paper. Both types of pulp are passed through refiners where the pulp is cut to increase its surface area. A mix of softwood and hardwood pulp slurry is fed into the blend chest where it is diluted and mixed for approximately 12 minutes. The pulp is then fed into the machine chest for another 12 minutes of mixing. These steps are essentially batch processes. The pulp is then pumped from the machine chest and is continuously fed into the headbox after undergoing various different treatments such as dilution and cleaning. The headbox spreads the stock on the wire. Excess water is drained into the silo and reprocessed. Presses are used to mechanically remove a portion of the water remaining in the paper. The remaining water is removed as the paper is dried on hot cylinders. Finally, the paper is compressed in a calender stack and wrapped on a reel. Festo Didactic

10 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Outline Batch process Batch process Continuous process Hardwood pulp Refiner Softwood pulp Headbox wire Calender Reel Refiner Dryers Silo ph process Blend chest Machine chest White water chest Figure Paper making process. PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Tuning the controller for batch ph process control Controlling a batch ph process Neutralizing the process water Curve analysis PROCEDURE Set up and connections Before using ANY of the chemicals provided with the ph Process Control Training System. Read the chemical MSDS and wear the appropriate personal protective equipment. 1. Use the piping and instrumentation diagram (P&ID) shown in Figure 1-24 to connect the equipment. If your training system is a Series 3532, use Figure 1-25 to position the equipment correctly on the training system frame. If your training system is a Series 3531, use Figure 1-26 to position the 28 Festo Didactic

11 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure equipment correctly on the training system frame. Use the basic setup presented in the Familiarization with the Training System manual. Table 1-2 lists the equipment you must add to the basic setup in order to set up your system for this exercise. Table 1-2. Equipment required for this exercise. Name Part number Identification Controller * AIC Volumetric flask Scopulla Sodium hydroxide solution 1.0 N Hydrochloric acid solution 1.0 N Phenol red Graduated cylinder Latex gloves Pipette Safety glasses ph transmitter AIT Metering pumps Paperless recorder UR Chemical tanks Festo Didactic

12 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Base ( solution) Acid ( solution) Figure P&ID. 30 Festo Didactic

13 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Figure Setup (series 3532). Festo Didactic

14 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Figure Setup (series 3531). 2. Wire the emergency push-button so that you can cut power in case of an emergency. The Familiarization with the Training System manual covers the security issues related to the use of electricity with the system as well as the wiring of the emergency push-button. 3. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup that is not before step Connect the ph transmitter to the controller input and the metering pump delivering acid to the controller output. Be sure to include the recorder in your 32 Festo Didactic

15 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure connections. On channel 1 of the recorder, plot the output signal from the controller. On channel 2, plot the signal from the transmitter. Be sure to use the analog input of your controller to connect the ph transmitter. Figure 1-27 shows the connections from the different devices to the recorder. Analog input In1 Out1 Ch1 Ch2 24 V Analog output Figure Connecting the equipment to the recorder. Hydrochloric acid is corrosive and harmful if swallowed. Do not breathe the vapor. Avoid eye and skin contact. When handling, wear chemical-resistant gloves, chemical safety goggles, and a lab coat. Refer to the MSDS for more details on this product. Sodium hydroxide is corrosive and harmful if swallowed. Do not breathe the vapor. Avoid eye and skin contact. Avoid prolonged or repeated exposure. When handling, wear chemical-resistant gloves, chemical safety goggles, and a lab coat. Refer to the MSDS for more details on this product. 5. For this batch ph process, one of the chemical tanks must be filled with a solution of 0.08 mol/l of sodium hydroxide and the other chemical tank with a solution of 0.08 mol/l of hydrochloric acid. Make sure there is enough solution in both tanks to perform the exercise. If not, make some more. Consult the Measurement manual for a detailed procedure to prepare these solutions. 6. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive. Be sure to follow the instructions in the Familiarization with the Training System manual as well. Festo Didactic

16 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure f All unused male adapters on the column are capped and the flange is properly tightened. The hand valves are in the positions shown in the P&ID. The chemical tanks are filled with the appropriate solutions and are carefully labeled. You are wearing the appropriate PPE. The vent tube is properly installed. The controller is properly connected to the ph transmitter and the metering pump delivering acid. The paperless recorder is set up and configured to record the output of the ph transmitter and the output of the controller. 7. Ask your instructor to check and approve your setup. 8. Power up the electrical unit, this starts all electrical devices. 9. Make sure the ph probe is properly inserted into the connection port on the process workstation. 10. On the recorder, make sure to plot in units of 0% to 100% the controller output (4-20 ma) connected to channel 1 and the transmitter output (0 ph to 14 ph) connected to channel Configure the metering pump delivering the hydrochloric acid solution so that its delivery rate goes from 0% to 100% when the output signal from the controller varies from 4 ma to 20 ma. 12. Configure the ph transmitter so that its output varies from 4 ma to 20 ma when the ph value varies from 0 to Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate, up to 50% of the maximum flow rate that the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair all leaks. 34 Festo Didactic

17 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure 14. Start the pump and set the drive speed to 30 Hz. This setup uses a telescopic pipe which can extend if the pump runs at a too high speed. Be sure to set the drive speed to a maximum of 30 Hz and secure the tubing as much attach brackets, Model 85444, as possible. 15. Fill the column up to 20 cm (8 in) of water, then close HV1 and open HV6 to put the process workstation into recirculation mode. 16. Remove one of the caps from the top of the column. Using a funnel, add about 20 ml of phenol red to the process water. Add more phenol red for more contrast if desired. Tuning the controller for batch ph process control 17. Set the controller to PI mode and configure it with the parameters shown in Table 1-3. Be careful to convert these parameters for your controller. Some controllers use proportional band instead of the controller gain or reset rate instead of integral time (or reset time). Table 1-3. Controller parameters. Parameter Value Controller gain ( ) 4.0 Integral time ( ) 48 s (0.8 min) Set point 40% 18. Using the metering pump connected to the chemical tank containing the solution of 0.08 mol/l of sodium hydroxide, start injecting alkaline solution into the process water with the metering pump delivery rate set to 25% of its maximum delivery rate. 19. Set your controller to automatic mode. 20. The parameter of Table 1-3 are starting values, you may have to fine tune your controller to refine the control of your loop. Use the PID tuning chart in Figure 1-20 and the trial-and-error method to fine tune the control of the batch process. 21. Since alkaline solution is constantly added, this batch process is more difficult to control when the ph is greater than the set point than it is to control when the ph is less than the set point. Therefore, to fine tune the controller, make changes in the set point from 40% to 50% or from 50% to 60%. Festo Didactic

18 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure 22. As you experiment with fine tuning the controller, make sure to adjust the level in the column to 20 cm (8 in) by opening HV Curves from a process with a well-tuned controller are shown below. Figure 1-28 shows a step change in the set point from 40% to 50% and Figure 1-29 shows a step change in the set point from 50% to 60% ph (%) Meteringpump(HCl) Controller output 60 Percentage (%) Percentage (%) Set point change Time (s) Figure Step change in the set point from 40% to 50%. ph ph (%) Meteringpump(HCl) Controller output Set point change Time (s) Figure Step change in the set point from 50% to 60%. 36 Festo Didactic

19 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Controlling a batch ph process 24. Once the controller is properly tuned, it is time to test your ph loop against a perturbation in the process. First, make sure to adjust the level in the column to 20 cm (8 in) by opening HV Next, make sure the controller set point is set to 40% and that the ph value of the process water is stable close to this set point. 26. When ready, change the delivery rate of the metering pump injecting alkaline solution from 25% to 50% and observe on the trend recorder how the controller reacts to this perturbation. 27. Note the time and date displayed the recorder. You will need this information to retrieve the data from the recorder at the end of this exercise. 28. If the controller is properly tuned, the ph value of the process water should stabilize within 10 minutes. If the ph does not stabilize, fine tune the controller once more and try again the Controlling a batch ph process section. The ideal tuning parameters can vary from one setup to another depending on the type of water used, the delivery flow rate of the metering pumps, the temperature, the quantity of water in the column, etc. Therefore, be patient and systematic while tuning your controller. 29. Once the process loop reacts appropriately to the perturbation in the alkaline flow rate described in step 26, try decreasing the delivery rate of the metering pump injecting alkaline solution from 50% to 25%. Again, observe how the controller reacts to this perturbation on the trend recorder. 30. Note the time and date displayed on the recorder. Neutralizing the process water 31. The process water contains a strong acid solution now. Therefore, it is very important that you neutralize the process water before draining the system. 32. Since the controller is now properly tuned, change the controller set point to 50% (i.e., a ph value of 7) and wait for the process to stabilize at the set point. 33. When the ph has reached a value between 6.5 and 7.5, it is safe to drain the system. Open HV4 and HV5 to drain the water from the column into the main tank of the process workstation. Festo Didactic

20 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Procedure Curve analysis 34. Follow the procedure in the Familiarization with the Training System manual to transfer the data from the paperless recorder to a computer. 35. Stop the system, turn off the power, and store the equipment. Do not forget to rinse the ph probe and store it into a storage solution as described in the Familiarization with the Training System manual. 36. From the data retrieved from the recorder, extract the data related to the perturbation induced in step 26. Plot the response curve to the perturbation using spreadsheet software. 37. From the data retrieved from the recorder, extract the data related to the perturbation induced at step 29. Plot the response curve to the perturbation using spreadsheet software. 38. Compare the response curve to the perturbation in the flow rate of alkaline solution from 25% to 50% to the response curve to the perturbation in the flow rate of alkaline solution from 50% to 25%. Which of the two types of perturbations seems to require the more aggressive control? Why? 39. Is the process is easier to control when the ph is below or above the set point? Why? 40. From what you have observed in this exercise. Describe how the process loop reacts if the controller gain is too small. 38 Festo Didactic

21 Ex. 1-1 PID Control of a Batch ph Process with a Strong Acid Conclusion CONCLUSION In this exercise you learned what a batch ph process is and how to tune such a process using the trial-and-error method. REVIEW QUESTIONS 1. What is the advantage of adding integral action to a proportional control scheme? 2. Why is on-off control not efficient in the previous procedure? 3. Why does the trial-and-error method proceed with a factor of two change at every iteration? 4. What happens if you increase the parameter in a PI control scheme? 5. Referring to Figure 1-20, explain how to set the gain and integral time of the controller in a process where it is essential that the controlled variable stays continuously below the set point. Festo Didactic